Modelling the London Basin and North Downs Aquifer

Case Study

We were commissioned by the Environment Agency to develop groundwater models of the London Basin and North Downs aquifer. A conceptual groundwater model was developed over the period November 2008 to November 2009. The development of a numerical groundwater model commenced in March 2011 and was completed in March 2012. The model has since been used for a variety of purposes by the Environment Agency and other stakeholders. 

 

A summary poster of our work on the conceptual model, presented to the Geological Society Thames Region Group in October 2010, can be viewed here.

Figure 1: Outcrop Geology of the Study Area
Figure 1: Outcrop Geology of the Study Area

The Chalk and Thanet Sands aquifer (Figure 1) provides the main groundwater resource in the London area, supporting significant abstraction for a variety of uses including public water supply. Beneath London the aquifer is confined by clayey strata of the Lambeth Group and London Clay.

 

As a consequence of historical abstraction, a regional cone of depression developed beneath Central London, in the centre of which the Chalk aquifer developed an unsaturated zone (see Figure 3 below). But since 1960, the reduction in pumping from industrial decline resulted in the groundwater levels rising over most of the area. This recovery would potentially cause issues with the structural integrity of infrastructure, which has led to a strategy of abstraction management to control groundwater levels in the area, a strategy known as GARDIT (General Aquifer Research Development and Investigation Team) – see www.groundwateruk.org/Rising_Groundwater_in_Central_London.aspx for details.

 

Aquifer recharge schemes have been developed in the areas where historical abstraction caused the development of an unsaturated zone in the Thanet Sands, such as along the River Lee (NLARS – North London Aquifer Recharge Scheme). The Environment Agency monitors the situation and produces an annual review of groundwater levels.

Figure 2: Structural Controls on Groundwater Flow
Figure 2: Structural Controls on Groundwater Flow

Conceptualisation

The London Basin aquifer is one of the most densely investigated and data-rich groundwater bodies in the UK. Following consolidation and analysis of the available data and a comprehensive literature review, we developed a detailed conceptual understanding of the key hydrogeological processes which occur within the London Basin aquifer. Conceptualisation focused on understanding and quantifying the following key aspects of the hydrogeology of the aquifer:

  1. Using the most modern interpretation from the British Geological Survey of geological layering and structure to understand the geometry of the aquifer units – see
    http://www.bgs.ac.uk/research/engineeringGeology/urbanGeoscience/londonAndThames/faultModelling.html for details.
  2. Understanding the hydraulic properties of the layered aquifer. Historical interpretations of the Chalk transmissivity distribution were combined with recent pumping tests and depth of burial information to prepare an initial transmissivity map for the model.
  3. Understanding groundwater level distributions to identify low permeability barriers (mainly identified as faults or fold axis as shown in red in Figure 2 above), and to map areas where the water table was below the base of aquifer units. Figure 3 below shows the change in water table in relation to geological formations in the Central London area over the period 1820 to 2007.  This shows the historical extent of dewatering in the basin and subsequent recovery.
  4. Quantifying recharge to the North Downs, which contributes almost a half of the water to the basin; and modelling flows from the springs at the foot of the dip slope which form the upper reaches of the Rivers Hogsmill, Wandle and Ravensbourne.
  5. Quantifying other flows into the confined basin, which contribute to most of the abstraction yield. The key source is the unconfined aquifer of the Chilterns, the data for which we obtained from other Environment Agency groundwater models.

Recommendations were then made for the development of a numerical groundwater model to simulate the hydrogeology of the London Basin.

Figure 3: Historic Levels of Abstraction
Figure 3: Historic Levels of Abstraction

Modelling

We developed a numerical model of the London Basin and North Downs aquifer in MODFLOW. From the original conceptual study, the model area was extended eastwards to the estuary of the River Medway. The key technical challenges were:

  1. The extremely large model area:  five hydrogeological layers are represented; the active model covers an area of 3175 sq km; the overall model period is 1810 to 2007, with ten-day stress periods from 1965.
  2. The model represents a highly stressed system, requiring the development and partial recovery of a regional cone of depression. Such groundwater level decline requires de-saturation of model layers which is inherently unstable to then re-saturate using standard MODFLOW codes. To overcome such stability concerns, a novel version of MODFLOW in which layers do not (numerically) dry out was used(1). Complex geology, particularly at geological outcrop boundaries, can also lead to instability and considerable work was required to translate this geological pattern into a numerically sound model.
  3. The model is being used to accurately represent groundwater levels in diverse hydrogeological environments; namely, a confined basin and unconfined Chalk downland, both with order-of- magnitude transmissivity variations. Figure 4 below shows the final transmissivity distribution for the model area.
  4. In the confined aquifer there is equivalence between the effects of linear barriers versus low permeability. Model testing, using manual and automated algorithms (such as PEST) was used to investigate aquifer transmissivity within the confined aquifer, with special emphasis on evaluating the role of faults in controlling drawdown and recovery rates.

 

 

Figure 4: Distribution of Transmissivity
Figure 4: Distribution of Transmissivity

Impact

We created the most up-to-date and accurate model of the London Basin and North Downs aquifer. The conceptual and numerical models aid the Environment Agency and other stakeholders in managing water resources of the basin, comprising tools which enable sound science-based regulatory decision making to quantify the sustainable balance of inflows and abstraction in the aquifer.  It has been used, for instance, to quantify impacts of groundwater abstraction on flows in watercourses, to estimate the potential impacts of climate change on aquifer recharge and deployable output and to optimise the use of artificial recharge schemes.

For further information on this project, please contact Mike Streetly, Water Group Director.

 

 

1. Note
As described by John Doherty in the article ‘Improved Calculations for Dewatered Cells in MODFLOW’ published in the academic journal Groundwater.
Citation: Doherty, J. (2001), Improved Calculations for Dewatered Cells in MODFLOW. Groundwater, 39: 863– 869. doi: 10.1111/j.1745-6584.2001.tb02474.x
URL: http://onlinelibrary.wiley.com/doi/10.1111/j.1745-6584.2001.tb02474.x/abstract.